U.S. Patent Documents:
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Hamm, Russell O., xe2x80x9cTubes Versus Transistorsxe2x80x94Is There an Audible Differencexe2x80x9d, Presented Sep. 14, 1972, at the 43rd Convention of the Audio Engineering Society, New York.
1. Field of the Invention
The present invention relates to the field of electronic amplifiers. More particularly, the present invention relates to the use of vacuum tubes with low supply voltage to provide amplification of musical electronic signals.
2. Description of The Prior Art
Russell O. Hamm, in his brilliant paper xe2x80x9cTubes Versus Transistorsxe2x80x94Is There an Audible Differencexe2x80x9d, says: xe2x80x9cVacuum-tube amplifiers differ from transistor and operational amplifiers because they can be operated in the overload region without adding objectionable distortion. The combination of the slow rising edge and the open harmonic structure of the overload characteristics form an almost ideal sound-recording compressor.xe2x80x9d He also states that vacuum tube circuitry only makes a definite audible difference when it is used at the mechanical-electrical interface.
Vacuum tubes are rarely used along with transistors, so called hybrid circuits, because of the different supply voltage requirements. Power supplies for vacuum tubes are generally much more expensive, and dangerous, because of the need fort high voltages and a separate filament heater voltage.
It would be an improvement over prior art to develop a hybrid circuit that uses a single low voltage transformer, for example 6 to 24 volts AC, to power the heater filament(s) and to produce a common B+ load voltage for the vacuum tube and transistor devices, for example 12 to 48 volts DC. However, vacuum tubes operated at low voltage typically have low gain due to increased plate resistance and resulting lowered transconductance. The few cases of prior art that attempt to solve the low transconductance problem use various transistor devices that do not fully preserve the pleasing harmonics and overload compression of vacuum tubes.
The present invention is a hybrid vacuum tube and JFET audio amplifier, hereinafter called TIJO (for Tube Input JFET Output), using a simple, low voltage power supply. The present invention solves the low transconductance problem associated with operating vacuum tubes at low voltages by connecting the tube output to a JFET (junction field effect transistor) while preserving the pleasing harmonics and overload compression of the vacuum tube.
The primary objective of the TIJO is to provide a transducer interlace audio amplifier incorporating a device that produces low order harmonics when overdriven. Said transducer may be a microphone, guitar, or even a digital to analog convertor. Vacuum tubes produce low order harmonics where each subsequent harmonic (higher frequency) is much lower in level than the previous harmonic. This characteristic is most relevant to the input stage which is connected directly to the transducer (via a coupling capacitor) as this is where transient peaks are most likely to be encountered.
A second objective of the TIJO is to disregard the standard practice of using negative feedback to reduce distortion and increase bandwidth, but instead celebrate the native response of the amplifying device by using zero (no) feedback. Also the use of operational amplifiers (op amps) is completely avoided as they are too clean and sterile sounding, i.e. they are extremely low distortion and have very wide bandwidth, and thus are very accurate but not very musical.
A third objective of the TIJO is to simplify the power supply by using a single voltage transformer to feed the filaments and produce a single low voltage B+. This is done with a 12 Volt AC transformer, a voltage doubler, and a zener regulator to provide about 30 Volts DC to the tube. This also enhances usability by allowing external placement of the tube as no high voltages are present.
The fourth objective of the TIJO is to provide a line driver audio amplifier that interfaces well with a vacuum tube operated at low voltage by presenting a high e to the tube output to minimize loading. The JFET is electronically and harmonically similar to the vacuum tube, except that it typically operates at lower voltages and higher currents, thus being optimum for the output stage to drive difficult loads.
The fifth objective of the TIJO is to allow the vacuum tube to be easily changed to various types including the 12AX7, 12AU7 and 12AT7. The different tubes provide different gains, plate resistances, bias responses, and, in general, overdrive differently to produce different tonal palettes.
Described generally, the TIJO comprises a vacuum tube having at least one grid for receiving an input signal, a plate run at low voltage for delivering a plate voltage responsive to the input signal, and a cathode connected to ground. The TIJO also comprises a JFET (Junction Field Effect Transistor) having a P-type region, or gate, diffused into an N type channel with a source or drain at each end for receiving an input signal at the gate from the tube with minimal loading and responding to said input signal by varying the resistance between source and drain, hence converting input voltage into output current. Both the vacuum tube and the JFET are supplied from a common low voltage B+. The TIJO utilizes zero feedback and is non-inverting. The vacuum tube is mounted externally. There are various other devices to support the TIJO circuit like load resistors, coupling capacitors, and bias resistors which follow standard practices from prior art.